Factors Affecting Microscope Resolution
1. Color difference
Chromatic aberration is a serious defect in lens imaging. It occurs when polychromatic light is the light source, and monochromatic light does not produce chromatic aberration. White light is composed of seven kinds of red, orange, yellow, green, blue, blue, and purple. The wavelengths of each light are different, so the refractive index when passing through the lens is also different. In this way, a point on the object side may form a color spot on the image side.
Chromatic aberration generally includes positional chromatic aberration and magnification chromatic aberration. Positional chromatic aberration makes the image appear blurred or blurred at any position with color spots or halos. And magnification chromatic aberration gives images with colored fringes.
2. Spherical difference
Spherical aberration is the monochromatic aberration of an on-axis point and is due to the spherical surface of the lens. The result of spherical aberration is that after a point is imaged, it is not a bright spot, but a bright spot with a bright middle and gradually blurred edges. This affects the image quality.
The correction of spherical aberration is often eliminated by lens combination. Since the spherical aberration of convex and concave lenses is opposite, convex and concave lenses of different materials can be selected to be glued together to eliminate. In the old model microscope, the spherical aberration of the objective lens is not completely corrected, so it should be matched with the corresponding compensating eyepiece to achieve the correcting effect. Generally, the spherical aberration of new microscopes is completely eliminated by the objective lens.
3. Coma
Coma is a monochromatic aberration of off-axis points. When the off-axis object point is imaged with a large-aperture beam, the emitted beam passes through the lens, and no longer intersects a point, then the image of a light point will get a comma shape, like a comet, so it is called "coma".
4. Astigmatism
Astigmatism is also an off-axis point monochromatic aberration that affects sharpness. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam is inclined greatly, causing astigmatism after passing through the lens. Astigmatism makes the original object point become two separate and mutually perpendicular short lines after imaging, which form an elliptical spot after being integrated on the ideal image plane. Astigmatism is eliminated through complex lens combinations.
5. Field song
Field curvature is also known as "image field curvature". When the lens has field curvature, the intersection of the entire beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire phase cannot be seen clearly during the microscopic examination, which makes observation and photography difficult. Therefore, the objective lens of the research microscope is generally a flat field objective lens, which has corrected the field curvature.
6. Distortion
In addition to the field curvature, the various aberrations mentioned above all affect the clarity of the image. Distortion is another property of phase difference where the concentricity of the beam is not destroyed. Therefore, the sharpness of the image is not affected, but the image is distorted in shape compared to the original object.
(1) When the object is located beyond the double focal length of the object side of the lens, a reduced inverted real image is formed within the double focal length of the image side and outside the focus;
(2) When the object is located at twice the focal length of the object side of the lens, an inverted real image of the same size is formed on the double focal length of the image side;
(3) When the object is located within twice the focal length on the object side of the lens, but outside the focus, an enlarged inverted real image is formed beyond the double focal length on the image side;
(4) When the object is located at the focal point of the object side of the lens, the image side cannot be imaged;
(5) When the object is located within the focal point of the object side of the lens, there is no image formation on the image side, and an enlarged upright virtual image is formed on the same side of the lens object side farther than the object.
Resolution The resolution of the microscope refers to the minimum distance between two object points that can be clearly distinguished by the microscope, also known as the "discrimination rate". Its calculation formula is σ=λ/NA where σ is the minimum resolution distance; λ is the wavelength of light; NA is the numerical aperture of the objective lens. The resolution of the visible objective lens is determined by the NA value of the objective lens and the wavelength of the illumination light source. The larger the NA value, the shorter the wavelength of the illumination light, the smaller the σ value, and the higher the resolution. To increase the resolution, i.e. reduce the value of σ, the following measures can be taken:
(1) Reduce the wavelength λ value and use a short wavelength light source.
(2) Increase the n value of the medium to increase the NA value (NA=nsinu/2).
(3) Increase the aperture angle u value to increase the NA value.
(4) Increase the contrast between light and dark.
